What is the function of "removing doubles UV"
This is a very useful function when making textures for game assets. 

When modeling game assets you usually get a lot of similar faces that could use the same texture space, so you want to align them on the same spot. But to save even more space, it's a good idea to use the "pack islands" function. 

If you haven't merged the UV's with the "remove doubles UV" function, they will get separated again and moved to different parts of the map.